One of the characteristics of a truly sustainable garden is that it produces at least some of its own seed. This is most often done when gardeners select, harvest and store seeds until the proper time for planting the following year. But some self-seeding crops produce seeds so readily that as long as you give them time to flower and mature, and set seed, you will always have free plants growing in your garden. You can simply let the seeds fall where they are, or toss pieces of the seed heads into the corners of your garden, or whichever area you want them in — no harvesting, storing or replanting required. With most self-seeding vegetables, herbs…